1. Field of the Invention
The present invention relates to a yttrium iron garnet (YIG) crystal oscillator circuit. More particularly, the present invention relates to control of loop gain in the YIG oscillator circuit to minimize FM noise and harmonics.
2. Description of the Related Art
A YIG crystal resonates at a microwave frequency when placed in a magnetic field. To create a tunable YIG oscillator circuit, a reactive component or YIG resonator is first formed by placing a YIG sphere in the air gap of an electromagnet. Current is then applied to the windings of the electromagnet using an active device such as a field effect transistor (FET) or bipolar junction transistor (BJT) to change the magnetic field as desired in order to obtain a desired frequency of oscillation. The frequency of oscillation of the YIG crystal is related only to the strength of the magnetic field applied and not to the dimensions of the sphere. General background on YIG crystals and circuits using YIG crystals can be found in J. Helszajn, "YIG Resonators and Filters," John Wiley & Sons (New York: 1985), incorporated herein by reference.
The YIG oscillator typically operates at a steady-state power level which is arrived at when the "loop gain" for the YIG oscillator is reduced to unity through gain compression. Loop gain is determined by the reflection coefficient of the YIG resonator multiplied by the reflection coefficient of the active component. Gain compression occurs when an increase in the RF input power versus RF output power of the active component deviates from linearity. As shown in FIG. 1, when gain compression occurs, the reflection coefficient of the active component reduces as the RF input power level increases.
To achieve start-up, loop gain must be greater than one. If the initial loop gain is large compared to one, the amount of gain compression required for the oscillator to reach steady-state oscillation will be large. Significant compression causes the oscillator to operate in a very non-linear manner, causing high FM noise levels, and high harmonic distortion, the former due to up-conversion of base-band active component noise.
To minimize compression, and thus reduce noise, initial loop gain is typically minimized, yet the initial loop gain must remain greater than one to ensure start-up. With an oscillator operating over a broad bandwidth, keeping the initial loop gain very close to one over the entire frequency band is difficult. Changes in ambient temperature and component drift over time further affect control of the loop gain making a broadband design even more difficult.